Immunocytochemistry has been a powerful tool in research in cell biology. It allows the precise localization of individual proteins to cell organelles and--when carried to the electron microscope level--even to specific subcompartments thereof. It provides information on localization of proteins in intact cells and therefore is essential in that it complements cells fractionation and biochemical approaches. To apply immunocytochemical methods at the electron microscope level is technically demanding and requires extensive instrumentation and highly skilled and experienced personnel. The purpose of Core B is to provide this expertise and make it possible for all member of the Program Project to apply these techniques in their program-related research. Personnel associated with the Core have considerable experience in all major approaches current used in immunocytochemistry involving immunofluorescence, immunoperoxidase techniques, and immunogold labeling of ultrathin cryosections. The Core Director has used these techniques in her research over 15 years, and Mr. McCaffery, the Core's Technical Director, has had over 15 years experiences in this area and has co-authored a number of publications, including 10 during the current funding period using these techniques. During the last four years under the auspices of the Program Project, we have operated the facility as a state-of-the-ar Core to serve program members. The Core has been heavily used by all Project leaders as attested to by the fact that 25 publications have utilized core facilities and/or personnel. Availability of the expertise of the expertise of the core has provided new information that led to new research direction for the Project Leaders.